The Computations of the Composing Brain: Cross-modal Generality and Computational Specificity

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One of the most impressive aspects of human cognition is our ability to produce and comprehend an infinite range of expressions, built from a finite set of lexical items in our long-term memory. Although composition is a defining characteristic of human language, its neurobiology is still largely uncharacterized. Specifically, current models lack computational detail as to the specific functions executed by various "combinatory brain regions." Consequently, rehabilitation strategies for brain damaged individuals suffering sentence processing problems are similarly generic, not targeting specific computations. Second, while composition during comprehension has been studied for decades, we know almost nothing about composition in language production, mostly due to methodological reasons: movement artifacts during talking compromise almost every type of brain data. With funding from the National Science Foundation, Dr. Liina Pylkkänen and her research team will carry out the first comprehensive investigation of the neurobiology of linguistic composition that simultaneously strives both for computational specificity, assessing specific functional hypotheses emerging from theoretical linguistics, as well as cross-modal generality, systematically testing the same stimuli across both comprehension and production. To overcome the methodological hurdles of studying production, this research uses short combinatory expressions (e.g., 'grey cat' or 'eats meat'), which can be fully planned before articulation commences. Brain activity will be monitored millisecond by millisecond during this artifact-free planning stage with magnetoencephalography, which offers an unparalleled combination of spatial and temporal resolution. This project primarily uses a very simple word-picture matching task, which is easy enough for future application in clinical populations and children. A specific focus of this work is the computations of the left anterior temporal lobe (LATL), a brain region that is atrophied in semantic dementia, a degenerative disease characterized by loss of semantic memory. The project will investigate whether the roles of the LATL in composition on the one hand, and in semantic memory on the other, are neurally distinct or instances of a unified process of semantic specification. These findings could be transformative for our understanding of both composition and semantic memory, in the healthy brain as well as in dementia.